Exemplary embodiments of the present invention relate to a camshaft arrangement for a driving mechanism, in particular for a motor vehicle engine. The camshaft arrangement has two shafts disposed coaxially one inside the other, wherein a hollow outer shaft and an inner shaft are disposed so as to be able to rotate with respect to each other. The two shafts each support a plurality of cams, wherein the outer shaft cams supported by the outer shaft are non-rotatably attached to the outer shaft, while the inner shaft cams supported by the inner shaft are non-rotatably attached to the inner shaft. If the inner shaft and outer shaft are rotated oppositely to each other, the cams are adjusted with respect to each other.
Such adjustable camshaft arrangements are increasingly being used in valve-controlled internal combustion engines to influence the control times of the valves of the engine in a targeted manner with respect to output and torque generation, fuel consumption and exhaust gas emission. These camshaft arrangements are, for example, built-up camshaft systems with an outer shaft and an inner shaft disposed coaxially therein. The two shafts can be rotated with respect to each other using an adjustment device. Cam elements which are alternately fixedly connected to the outer shaft and mounted so as to be able to rotate about the outer shaft are disposed on the outer shaft. The rotatably mounted cam elements are fixedly attached to the inner shaft but can rotate relative to the outer shaft by a defined circumferential angle. This is achieved, for example, by the inner shaft cam elements being fixedly connected to the inner shaft using a connection element, wherein this connection element protrudes with clearance through an aperture in the outer shaft, which enables rotation of the inner shaft and therefore of the inner shaft cams by the defined circumferential angle.
The rotation of the inner shaft with respect to the outer shaft is effected, for example, using a phase shifter which permits an adjustment of the outer shaft cams fixedly connected to the outer shaft with respect to the inner shaft cams fixedly connected to the inner shaft or vice versa, in order thus to achieve a phase shift of the valve control times.
In order to produce a fixed connection between the inner shaft and the associated inner shaft cam elements, these cams and the inner shaft conventionally have bores, into which corresponding connection elements are inserted. These can be pins, bolts or screws. The cam elements can be appropriately designed to permit insertion of a connection element.
German laid-open document DE 197 57 504 A1, for example, discloses for this purpose a camshaft with such a connection element in the form of a cylindrical pin. The cylindrical pin is inserted with a press fit into a through-bore in the inner shaft and with a clearance fit into the associated cam. The cam has for this purpose an outwardly-located cylindrical region in which a bore is located, into which the cylindrical pin is inserted. The cylindrical region is intended to ensure that the bore is located away from the contact surface of the cam in order in this way to produce an uninterrupted contact surface between the cam and the outer shaft.
Furthermore, German laid-open document DE 10 2005 004 976 A1 discloses a camshaft with cams which can be rotated oppositely to each other, in which a connection element is designed in two parts. A first part of the connection element is hollow and protrudes into a radial bore in the inner shaft. A second connection part is inserted into the hollow first connection part from the other side in such a way that it expands and an interference fit is produced. The two connection parts can also be designed, for example, as a screw with a complementary threaded bore.
European patent EP 1 362 986 B1 also discloses the mounting of an adjustable camshaft arrangement in which a hollow coupling pin is guided with a narrow fit into a bore in both the inner shaft and the outer shaft as well as into a cam. The outer diameter of the coupling pin is then enlarged by insertion of an inner pin in order to produce a press fit.
German patent DE 28 22 147 C3 further discloses a camshaft arrangement in which the connection between the inner shaft and cam is effected via an entrainer, which is attached to the inner shaft with a screw. The outer shaft has an aperture for passage of the entrainer. The entrainer further protrudes into an inner groove which is located at the joint diameter of the associated cam. The cam is pushed with the groove onto the entrainer from the side and secured in its axial position on the right and left with rings.
If the connection between the inner shaft cams and the inner shaft is arranged as a fixed connection owing to the alternating camshaft loading, a press fit of the connection elements is expediently employed both in the cam and also in the inner shaft. This is disadvantageous in the known connection elements, which have a constant cross-section in that the cross-section of the connection element required to achieve the press fit must be joined via a plurality of component bores in the cam and inner shaft. This can lead to undefined influencing of the pressing overlap, and therefore of the security of the press connection. In particular, joint connections of this type can lack sufficient connection firmness.
Exemplary embodiments of the present invention provide a built-up camshaft arrangement which is easy to fit together and offers secure connection with sufficient firmness between the inner shaft and inner shaft cam.
This object is achieved in accordance with the invention by a camshaft arrangement A camshaft arrangement for a motor vehicle engine, comprising a hollow outer shaft supporting a plurality of outer shaft cams, wherein the outer shaft cams are non-rotatably attached to the outer shaft and an inner shaft disposed coaxially inside the outer shaft, wherein the hollow outer shaft and the inner shaft are disposed so as to be able to rotate with respect to each other, and wherein the inner shaft supports a plurality of inner shaft cams that are each non-rotatably attached to the inner shaft by at least one connection element. The at least one connection element protrudes with clearance through an aperture in the outer shaft and is attached to one of the plurality of inner shaft cams that is rotatably mounted on the outer shaft and the at least one connection element is inserted into a receiver in the inner shaft so that a portion of the connection element protrudes out of the receiver, and the protruding portion is at least partially inserted into an aperture located at a joint diameter of the one of the plurality of inner shaft cams, wherein the aperture is open at least towards an end face of the one of the plurality of inner shaft cams, and the protruding portion has at least two opposite-lying side surfaces that lie with an interference fit against two corresponding inner surfaces of the aperture of the one of the plurality of inner shaft cams.
The camshaft arrangement according to the present invention involves a respective connection element that is inserted into a receiver in the inner shaft in such a way that a portion of the connection element protrudes out of the receiver, and that the protruding portion is at least partially inserted into an aperture which is located at the joint diameter of the respective inner shaft cam. This aperture is open at least towards an end face of the inner shaft cam, and the protruding portion has at least two opposite-lying side surfaces which lie with an interference fit against two corresponding inner surfaces of the aperture of the respective inner shaft cam.
In one aspect of the present invention the connection element is a feather key, and the receiver in the inner shaft is formed as a feather key receiver. In another aspect of the present invention the connection element has a stem and a head portion, wherein the stem is inserted into a receiver in the inner shaft, while the head portion is inserted at least partially into an aperture which is located at the joint diameter of the respective inner shaft cam. This aperture is open at least towards an end face of the inner shaft cam, and the head portion has at least two opposite-lying side surfaces that lie with an interference fit against two corresponding inner surfaces of the aperture of the respective inner shaft cam. Using the aperture that is open towards the end face of the inner shaft cam, the inner shaft cam can be pressed onto the head portion of the connection element in parallel with the longitudinal axis of the camshaft. In this way, the two interference fits of the stem/inner shaft and head portion/inner shaft cam cannot influence each other and the two interference fits can be produced reliably and with sufficient connecting firmness.
The connection element can have a stem and a head portion, wherein the stem is inserted with an interference fit into the receiver, while the head portion is at least one component of the protruding portion of the connection element. In one aspect of the present invention the head portion of the connection element is located completely outside the outer shaft. Alternatively, however, a head portion can also be located only partially within the aperture in the respective inner shaft cam and partially within the aperture in the outer shaft.
Furthermore, the head portion can also be located partially in the receiver in the inner shaft, extend through the aperture in the outer shaft and then into the aperture in the inner shaft cam. In this case the inner shaft can have a stepped bore as the receiver, into which the head portion of the connection element partially protrudes, while the lower region of the bore receives the stem of the connection element. This stepped bore is formed, for example, as a stepped cylindrical bore with two diameters. The larger, externally disposed diameter receives a portion of the head portion of the connection element. The portion of the head portion disposed in this widened portion of the stepped bore is adapted to the diameter of the widened region of the stepped bore so that a cylindrical head portion has a larger cross-section in this region. Thus, the loading of the connection element in this critical region is reduced because the increase in cross-section reduces the effective tension. The surface pressure in the contact region between the inner shaft and the head portion in the widened region of the stepped bore is also clearly reduced.
As an alternative to a stepped bore, a stepped aperture can also be provided as a receiver in the inner shaft. In contrast to a stepped bore with at least two cylindrical portions of different diameters, this stepped aperture has a cylindrical portion, in which the stem of the connection element is disposed with an interference fit, and a non-cylindrical portion which receives the non-cylindrical head portion. The non-cylindrical form advantageously ensures that a precisely positioned alignment of the head portion relative to the longitudinal axis of the camshaft arrangement is rendered possible so that the inner shaft cam can be placed precisely in the correct desired position. At the same time, the stem of the connection element can be cylindrical and inserted in a simple manner into the cylindrical portion of the stepped aperture. If the non-cylindrical portion of the stepped aperture is formed in such a way that the cross-section of this portion is larger than that of the cylindrical portion then, in addition to the advantage of the exact alignment of the head portion of the connection element, the advantages mentioned above in relation to the stepped bore, concerning a reduction in the tensions in the critical component region and a reduction in the surface pressure in the contact region between the inner shaft and head portion, are achieved.
Furthermore, the stem of the connection element can extend through the body of the inner shaft and into a second aperture in the outer shaft. An inner shaft cam can also be connected to the inner shaft by two opposite-lying connection elements in such a way that a torque transfer is ensured. The stems of two connection elements can also be inserted into a common receiver in the inner shaft.
The aperture can extend for passage of the connection element through the outer shaft over a portion of the circumference of the outer shaft in such a way that a movement of the connection element and therefore of the inner shaft relative to the outer shaft by an adjustment angle α is possible.
The head portion of a connection element can be formed at right angles to the stem portion of the connection element. Furthermore, the two side surfaces of the head portion and the two inner surfaces of the aperture can each extend in parallel or conically with respect to each other in order to make it easier for the connection part to be pushed into the aperture or for the inner shaft cam to be pushed onto the head portion and in order to produce a press fit. The two side surfaces of the head portion and/or the two inner surfaces of the aperture in the inner shaft cam can also have a profiled surface.
In both aspects of the present invention the aperture in the inner shaft cam can be formed as a groove extending over the whole width of the inner shaft cam. This aperture can be centrally located below the elevation of the inner shaft cam, and the connection element is preferably formed as a single-piece component.
One advantage of the first aspect of the present invention in which a feather key is inserted as a connection element is to be found in the fact that feather keys are inexpensive mass-produced components which are readily available. The feather key connection technique has been well mastered in terms of manufacturing technology. Furthermore, the whole cam width, i.e. the cam thickness in the axial direction, of the inner shaft cam for the formation of the press fit between the feather key and inner shaft cam can be used, whereby a particularly large joint surface for the formation of the pressing connection between the inner shaft cam and connection element is provided. This produces a particularly firm connection, whereby the radial construction height of the inner shaft cam is minimised in turn, which makes possible a saving in construction space and weight. This embodiment of the camshaft arrangement in accordance with the invention with a feather key can therefore be used in a particularly advantageous manner in smaller passenger car camshafts.
A further advantage of the present invention, and in particular of the second aspect of the camshaft arrangement in accordance with the invention, is in the fact that the cross-section of the connection element for the required interference fit of the connection between the connection element and the inner shaft or between the connection element and the inner shaft cam does not have to be joined over the whole connection element. Instead, an interference fit for the connection between the connection element and inner shaft is produced over the cross-section of the stem and, separate therefrom, over the side surfaces of the head portion an interference fit for the connection between the connection element and inner shaft cam is produced. This leads to an increased level of security in the respective interference fits.
Furthermore, an inner shaft cam is connectable in a simple manner to the inner shaft in that initially the connection element is inserted into the inner shaft and the inner shaft cam is then pushed onto the head portion of the connection element from the side.